Tufting machines that employ double needle bars are conventionally employed to tuft carpet backing or other substrate. The double needle bars generally feature a front and a back needle bar which are mounted on the same member. A row of needles is mounted to each bar, and the needles in the front row are typically staggered laterally with respect to the needles in the back row. (The needles may be mounted on the same bar.) The member carrying the bars may be actuated at appropriate intervals to cause the front row and back row of needles (and the ends of yarn threaded through them) to penetrate the substrate in order to form two rows of piles simultaneously. An arrangement of hooks is usually mounted on a rocker shaft below the substrate and actuated so that the hooks engage the yarn ends to form the piles as the needles retreat from the substrate.
Double needle bar tufting processes commonly tuft multiple colors of yarn ends in order to form colored pile patterns. For example, the back row of needles may be threaded with a first color and the front row with a second color, in a "two up" process. In a "three up" process, the leftmost needle in the back row may be threaded with a first color, the leftmost needle in the front row with a second color, and the next needle in the back row with a third color. The next needle in the front row carries the first color, the next needle in the back row the second, and the next needle in the front row the third as the yarn color order repeats itself along the rows of needles. "Four up" and "five up" threading patterns feature similar repetitive color orders.
Multicolored threading patterns such as those described above generate repetitive color patterns as the substrate is tufted. A substrate tufted with a non-shifted needle bar in the three up color order described above, for example, features a leftmost line of piles of the first color oriented in the machine direction, followed by a line of second color piles, followed by a third color line. The pile lines repeat the color order across the substrate perpendicular to the machine direction.
The double needle bar may be shifted or "indexed" left and right across the machine direction in order to insert piles of various colors into each line of piles. Thus the double needle bar may be actuated to tuft a front and back row, and then shifted right or left before tufting the next front and back row. The bar may be shifted right or left any number of times as desired according to a predetermined pattern in order to vary the color of piles, and the order in which the colors appear, in each pile line.
It is, however, conventional to index the needle bar the full distance between needles in a front or a back row (a "full gauge"). This is because the hooks which cooperate with the front row of needles are typically of different length than the hooks which cooperate with the back row of needles, so that each back needle must always assume the position previously taken by another back needle when the needle bar is shifted. Similarly, the front needles must always be aligned with a long hook and thus be shifted a full gauge.
Yarn patterns tufted with conventionally shifted double needle bars thus exhibit repetition in the machine direction and across the machine direction, although the repetition is less evident than in patterns generated using non-shifted double needle bars. As an example, a two up conventionally shifted double needle bar generates pile lines of alternating color across the machine direction whether or not the needle bar is indexed during tufting. A four up non-shifted double needle bar generates a repeat of four colors of pile lines, while full gauge shifting the bar successively left and right creates a repeat of two lines of piles, each formed of alternating first and third, and second and fourth, colors. The pattern repeats generated by three and five up arrangements are less evident, but are nevertheless present both in and across the machine direction.
The repetitive nature of patterns created by conventionally shifted double needle bar tufting is amplified when the tufted substrate is converted into carpet tile. In such applications, the substrate is typically cut in the machine direction and across the machine direction as the tiles are formed. A floor covering formed of such tiles may exhibit a "zipper" effect caused by the repeating pattern of colors in edge pile rows or lines on one tile being in alignment with the same repeating pattern on an adjacent tile. Such alignment accentuates the periodic repetition and alignment of dominant colors in the adjacent patterns, and thus highlights the seams between tiles. The effect is sometimes amplified across an entire floor formed of such tiles, as the eye integrates the aligned repetitive patterns across a longer distance. Furthermore, the effect is often more pronounced in tiles formed using a two or four up process, as distinguished from a three or five up process.